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Thread: STRANGE ODOR

  1. #1
    Ken Bates's Avatar
    Ken Bates Guest

    Default STRANGE ODOR

    The specific warm air, gas fired furnace is sold as CONCORD and made by ARMSTRONG. It is 6 years old. It is a 90% efficient condensing unit. I guess the type is called up-flow (circulated air) and down-draft (exhaust gases induced down by a motor driven fan to an area where the condensate drains out and then exits upwards via PVC exhaust vent to the exterior.)

    The 2 part heat exchanger consists of 4 ferrous steel clamshells on the top and stainless steel tubing at the base for ultimate heat extraction and condensing the water from the combusted methane gas.

    My only vantage point is via a 5X11 inch removable register cut into the plenum. I use a large glass mirror to view the exchanger. Everything looks clean and brand new.

    THE PROBLEM: My nose and my Tif combustible gas detector say something is wrong. The odor is best described as the smell you get when book (paper) matches fail to fully ignite. Sort of a Sulphur-like odor. Slightly acrid. A reliable CO detector shows nothing. It does show about 25 ppm at a test port drilled into the exhaust pipe a few inches above the furnace vestibule. Also, this burnt match odor cannot be detected when I smell the exhaust exiting from the vent on the exterior.

    OBSERVATIONS: I once saw a small spot of liquid on the floor pan of the blower compartment. As time goes on there are white, hard deposits accreting on the floor pan and in other areas. It appears that a liquid is dripping down from the heat exchanger area into the blower chamber. Also, some have made their way out of the plenum and dripped down the exterior part of the plenum where a flange was created to attach the plenum to the furnace cabinet. ( The humidifier is not attached to the plenum and is downstream on the main duct supply trunk and we do not have hard water with minerals and the problem occurred without water in the humidifier.)

    Simply said it appears that condensate is escaping and then being blown upwards and also evaporating and then leaving a hard white residue. However, I have taken an ounce of condensate and evaporated it and found no residue. The strong smell seems to occur at first firing in the morning (the t-stat is lowered at night)

    I once got to speak with an unfriendly Armstrong tech person who burbled something about a coating applied to the interior of heat exchangers. Anyone have any ideas? I have consulted a highly rated HVAC company and they found nothing. I suspect a seal failure where the round stainless exchanger tubes connect to a manifold that separates gas from condensate. But I have never seen this part of a furnace. This area is not visible or readily accessible. $1,200 to dismantle the furnace to get at the exchanger. The clamshell is very visible and looks perfect. No stains or white deposits on it, however, my theory is the very hot clamshell is evaporating the mist that is coming from a leak below and carries something dissolved in it that smells like half burnt book matches.

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  2. #2
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
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    Default Re: STRANGE ODOR

    Something could have fallen down into the heat exchanger which may be causing the smell.


  3. #3
    Join Date
    Mar 2007
    Location
    Plano, Texas
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    4,170

    Default Re: STRANGE ODOR

    Not much help since condensing furnaces are rare here, but have you cleaned the condensation drain port?
    I would suspect the slightly acidic condensate is backing up and then boiled off when the furnace fires.

    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

  4. #4
    Ken Bates's Avatar
    Ken Bates Guest

    Default Re: STRANGE ODOR

    To Raymond Wand,

    Good thinking. When I switched from a gravity oil fired octopus in 1977 the very competent and thorough installers made one boo-boo. They sealed the connections for the several ducts on the interior of the plenum with ordinary cloth gaffers tape instead of the aluminum stuff. It took many years for the tape and its adhesive to dry and then fall down bewteen the clamshells. And indeed there was a horrible odor. But I removed any remaining tape.

    To Jim Luttrall,

    Thanks for that idea. I'll pull some drain plugs and see if that is the cause.


  5. #5
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
    Posts
    5,005

    Default Re: STRANGE ODOR

    Fwiw, a number of years ago I had a client call me subsequent to them moving into the house. They wanted to know why I did not see the cracked heat exchanger on their 4 year old high eff. gas forced air furnace.

    Needless to say, being quick on keeping my clients happy I returned the following day. They explained to me they were not unhappy with me, as I had reported that the furnace should be serviced prior to close of title because of clues such as rust spot under the induced blower, and overall dirty condition. They failed to exercise my caveat, and in the process of having the furnace serviced prior to my arrival the heating tech pulled the furnace apart, and low and behold he found a child's toy on top of the heat exchanger, right were the crack was! The clients were happy as the heat exchanger was under warranty. How that toy got down into the heat exchanger kept everyone guessing because the duct work immediately above the exchanger made a right angle.

    Cheers,


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